He holds his views on life, law, right and wrong, as philosophy. You could say, as he's obviously highly intelligent, that he suffered a mental brake. Probably done by an over thinking of philosophy. He has suicidal tendencies, a lack of concern for his own life, and this was probably brought on by him thinking life is one big joke. He cracked the **** up, because he couldn't find a point or reason for people existing, even the universe.

Within that, the rules and decency of a normal society no longer applied to him. He couldn't go back to the way he was, if he ever was "sane" prior, of course.

Joker intended to take away the rules and decency away, as he thought it was what caused society from becoming like him. To him, it was all an apithany.
To him, without rules, society has no conscience. Everything will descend into madness, as that, he believes, is the true nature of people. Rules keep us grounded and sane, to the laws a decent society has layed down for us to follow.
Joker wants to eradicate all that, as he sees us as the ones who are crazy, not him.
That was his reason for living, imo. To break the human spirit, installed with the rules and morals of a decent society. Social restraints, that keep us held back from being who he believes us to be...and that is, monsters.

To me, Joker was incredibly philosophical. So much so, I believe he drove himself mad with his revelation and tried to kill himself, only to survive. This, in turn, left him with the scars in his face...resembling a smile.
He probably found it funny. The trauma in that, mixed in with his new views on life, that it's all just a joke, led him on that spiral.

To be honest, I believe, this Joker has crafted his look in order to hold a mirror up to society. To show them, that this is what we all are, but none of you don't realise it.
Batman took absurdity to a new level, and his life was fixated on battling crime and upholding the law. Something Joker already opposed. So, he was both inspired and obsessed with Batman. Taking upon himself to destroy this force that had both a mix of madness and sanity.

To Joker, he's not crazy. On the outside of the box, he's sane. We, inside the box, are the ones that are crazy.

He embraces chaos and madness, because he knows they are the tools that will break the chains of law and order that hold us back from being our true selves.

Joker knows that it's evil, but to him, it's what we all are deep down. He simply wants to establish himself as Emperor of the new world order he's showing us, a world hidden away by unnecessary rules, a concept he can no longer follow. He wants to be king of the jokers.

Because that's what life is to him, a joke, and we are nothing but clown's for the amusement of nobody.
Rules and sanity have prevented our eyes from being opened, and it's his mission to open our eyes to the world as he sees it.

Even then, he wouldn't call it his legacy. Because, nothing matters to him anyway. He's just an agent of Chaos, not a force above it.
If Jesus was the messenger of God, then Joker is the messenger of the Devil.

__________________"Every time you open that dvd box to watch the Dark Knight, it's actually a slice of baloney. the real movie is in your mind and Nolan performed an Inception on all of us." - tacit-ronin-